This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In the presence of water, aligned lipid membranes self-assemble in the vicinity of flat and smooth surfaces forming a smectic-C liquid crystal structure with water intercalated between the bilayers formed by the lipid molecules. By sandwiching hydrated lipids in between two substrates one can achieve several square-millimeter large defect-free monodomains of highly aligned lipid membranes. Using thin, x-ray transparent silicon-nitride windows as alignment substrate transmission SAXS experiments on such samples are possible. The small size and high intensity of the beam available at BL4-2 together with the large defect free domain size allows then to probe the structure in the plane of the membrane. In this project we investigated the change in the nearest neighbor distance between peptides inside the membrane in dependence on the thickness of the lipid and the peptide concentration. We find that the average distance between the proteins increases with increasing thickness of the bilayer. This effect is attributed to the increased membrane mediated repulsive interaction between the peptides due to the increased hydrophobic mismatch. We also found that a decrease in peptide concentration causes a larger than expected increase in the peptide-peptide distance, which could be caused by a concentration dependent boundary layer of lipids around the protein. We are currently working on a more detailed analysis of the data collected.